Protecting Your Pond in the Winter Months

What should be done to protect our ponds and their inhabitants against the ravages of winter? Before we can take the steps we need to protect our landscape or recreational pond, we should understand what actually goes on in the life of the pond during the winter season.

Fish kills in Ohio ponds are rare events considering the large numbers of ponds and small lakes located in the state. However, they do occur every year and some years are worse than others. Fish kills can result from a variety of causes but the most common causes are changing weather, amount of pond vegetation preesnt, and the resulting interaction of the two. The critical factor in the pond equation is the level of oxygen in the pond. Oxygen is essential to all aquatic life and should be maintained at levels of 4 ppm (parts of oxygen per million parts of water-milligrams per liter of water) or higher. The “Winter Oxygen Cycle” regulates oxygen levels in the pond during the winter months.

During winter, green plants and algae slow down their metabolism and die off, in response to the decrease in sunlight and water temperature. When this happens, the production of oxygen through photosynthesis is decreased. However, this does not mean that all oxygen levels in the pond are much lower in winter. In unfrozen ponds, for instance, high oxygen levels will occur during winter because the oxygen needs are less in cold water. Aquatic animal (primarily fish) metabolism and oxygen-consuming decomposition processes are greatly reduced in cold water. Cold water also contains more oxygen than the warmer summer water because more evaporation occurs in warm weather depleting oxygen levels. On the surface of unfrozen ponds, strong winter winds also keep the pond water circulating and continually add oxygen throughout the winter months. These factors combine to prevent winterkill in unfrozen ponds, even though plants are contributing only a small amount of oxygen in the winter.

Problems in the winter pond begin to occur when ice forms on the pond.

As ice forms, neither the wind nor contact with the air is able to contribute oxygen to the water. The only source of oxygen becomes that produced by the few remaining algae and plants. If the ice remains thin and clear, oxygen production by plants and algae can continue as enough sunlight will filter through the ice to allow photosynthesis to occur. This oxygen production can nearly compensate for that which is used up by respiration and decomposition that is continually going on. If the ice remains into mid-winter or thickens, oxygen levels will begin to decline. This is because each day, the plants and animals use slightly more oxygen than what is produced. At this stage though, the pond still has sufficient oxygen.

Serious problems develop when ice persists into late winter and thickens, or is covered by snowfall. The amount of sunlight reaching the plants is drastically reduced and so are oxygen levels. However, metabolism and decomposition continue in the fish population, so oxygen levels begin to drop and can approach levels that threaten the survival of the fish. In cold water, oxygen levels that remain at less than 2-3 ppm for an extended time will begin to kill fish. If the level drops to 1-2 ppm or lower throughout the pond, a complete fish kill will result.

There are several factors that influence the probability of a winterkill:

• Winter severity is the most important factor in determining the likelihood of a winterkill. A mild winter means little or no ice, and no winterkill caused by lack of oxygen. A harsh winter increases the chance of problems, due to thicker ice and snow cover. Four inches of snow on top of ice nearly eliminates sunlight penetration quickly reducing oxygen levels in the pond.

• Pond volume is important because the greater the volume of water in the pond, the less likely for winterkill to occur. This is why fish over-winter better in larger and/or deeper ponds. A one-acre pond that averages 6 feet deep will have more winter oxygen available than a one-acre pond that averages only 3 feet in depth.

• Decomposition of the amount of decaying organic matter that is present on the bottom of the pond is an important factor. Dead aquatic vegetation and tree leaves account for most of the organic matter that decomposes during winter. Ponds that have a very dense aquatic plant community in summer are the ponds that are most susceptible to winterkill during harsh winters.

• Fish biomass (the amount of fish in numbers and pounds) in the pond during winter also influences oxygen decline under the ice. Even though fish slow down their metabolism during the winter, they still require oxygen. A pond that contains many pounds of fish will experience a faster decline in oxygen than a pond with fewer pounds of fish. This is why fish farmers closely monitor ponds in winter since they are maintaining fish biomass at levels that greatly exceed a normal pond.

Pond owners should be most concerned during a harsh winter in which ice cover persists, there is considerable snow cover on the ice, the pond is small and shallow, and the pond contained a large amount of aquatic plants the previous summer.

Winterkill Prevention

Winterkills can be prevented in most cases, due in part to changes in weather conditions. Even though winters in Ohio tend to be harsh, a sudden warm spell will partially thaw a pond for a few days. Oxygen levels rebound quickly when a pond becomes ice-free. A single warm period of 2-3 days occurring at the right time can greatly reduce the possibility of a fish kill. There are also measures that a pond owner can take to help prevent a winterkill.

Prevention Activities

Limiting Plant Decomposition

• During construction of the pond, the inside slopes should be constructed to prevent excessive growth of aquatic vegetation. A slope that drops one foot in depth for every three feet of distance towards the center of the pond, or a 3:1 slope, should be maintained along most of the shoreline areas. This will limit the amount of shallow water where summer aquatic plant growth occurs. The reduced summer aquatic vegetation means less decomposition and results in higher oxygen levels under the ice in winter. Because deeper ponds experience winterkills less often, it is important to have a maximum depth of 10 -12 feet if possible. A good rule of thumb is ¼ of the pond’s surface area should be 8 feet deep or ½ of the pond’s surface area should be 6 feet deep.

• You can also obtain Ohio State University Extension Fact Sheets A-3-98, Controlling Filamentous Algae in Ponds, A-4-98, Chemical Control of Aquatic Weeds, and the Ohio Pond Management bulletin to learn more about strategies for controlling aquatic plants.

Aeration:

• The installation of an aeration system will keep a small area of the pond ice-free. Aeration not only adds oxygen to the water directly due to the bubbles and agitation, but the open area allows for considerable diffusion of oxygen into the pond from the air. The aeration system does not need to be run continuously all winter and can be turned on only when ice is forming on the pond. Leave it turned off when the pond is ice-free. Aeration can be used sparingly in winter to minimize ice cover.

Safety Note: Aeration during winter generally prevents safe ice for skating or ice fishing from developing anywhere on the pond.

Snow Removal

• If your pond is ice-covered and aeration is not an option, fish winterkills can often be prevented by simply removing some of the snow from the ice. About 25-50% of the pond surface needs to be kept free of snow to maintain enough light to allow sufficient photosynthesis to occur. Snow removal is really effective in small, shallow ponds, where the volume of water may not be sufficient to allow oxygen to last all winter.

Safety Note: Only remove snow when the ice is safe. Having ice that is at least a minimum of four inches thick is a good rule to follow before stepping onto the ice to remove snow.

Maintaining a healthy pond year-round will help protect your fish from a winterkill. Other sources for this article and more information on “Ohio Fish and Pond Management,” are also available through the Ohio Department of Natural Resources, Division of Wildlife, and the OSU Extension, as well as from the Avon Lake Department of Public Works at (440) 930-4101.